1. Field of the Invention
The present invention relates to interconnections between connectors and leads and, more particularly, to controlled impedance interconnections between micro-strips and micro-connectors, specifically, x-band coaxial connectors, for reducing distortion in the transmission of electrical signals.
2. Description of Related Art and Other Considerations
Distortion caused by transmission pathways is proportional to the pulse-repetition rate and switching speed of the signals, and to the length of the pathway. A major component of this distortion is caused by improper impedance matching. A circuit is said to be perfectly matched when both the source and the load at the opposite ends of the transmission pathway match the characteristic impedance of the pathway. When impedance discontinuities are present, unwanted reflections are created within the transmission pathway. These transmissions degrade the signal by effectively cancelling the transfer of energy from the signal source to the receiving circuit. These reflections arise when a signal encounters a sudden change in characteristic impedance somewhere along a transmission line. Two simple techniques are used to reduce these reflections: shortening the length of the transmission line or spacing the conducting lines more closely to minimize line bends. The disadvantage of the close spacing is that it greatly enhances the problem of crosstalk.
Crosstalk created by fields of radiation is propagated by electrical current flowing through transmission lines and inducing currents in other nearby conductors. Both electrostatic and electromagnetic fields produce unwanted interference in transmission lines.
Introduction of gallium arsenide monolithic technology into the digital-circuit arena creates instances in which microstrip lines are used for carrying pulses rather than microwave energy. Digital pulses are attenuated in transmission lines by shunt capacitance between traces through the dielectric, series resistance of the conductor, and dielectric losses. The resistive losses are aggravated by the skin effect, but vary only as the square root of the frequency. Conventional shielding, which completely encloses a transmission line within a grounded conductor, can reduce a large share of this type of distortion caused by electrical radiation; but extremely high signal frequencies tend to defeat such protective measures. One way to minimize crosstalk is to separate transmission lines by relatively large distances. However, design constraints and countervailing sources of distortion, which would be proportionally increased by larger line separations, militate against using this tactic to reduce the source of noise. Another simple means of reducing crosstalk is to form a helical arrangement of pathways by using two common insulated wires, which are twisted tightly together. Although the added path length brought about by the winding of the conductors adds time delays and other forms of distortion, radiation from each wire in such twisted pair substantially cancels that emitted by the other, and thus cuts down on crosstalk. Twisted-pair transmission lines are not used in subnanosecond applications.
A fixture, described in U.S. Pat. No. 4,672,312 (see also FIG. 1 of the drawings herein), solved the problem for testing of high-speed devices. A fixture, described in U.S. Pat. No. 4,825,155, was tested on a network analyzer with favorable results to 14 GHz.
Experimental test boards of the type disclosed in above-referenced U.S. Pat. No. 4,672,312 were fabricated and tested, and the test results thereof are discussed in the publication by Henry Takamine and Allan Lange, the inventors herein, entitled "Pitfalls in Fabricating Microstrip Test Boards for High-Speed GaAs ICs," Journal of Monolithic Technologies, July 1988, pages 22-27. Analysis determined that a major contributor to high discontinuities in the transmission of the signal and excessive voltage standing wave ratio (VSWR) occurred at the transitions from the coaxial connectors to the microstrips, and from other electromechanical connections in the test jig, as will become better understood from the following description.
Specifically, as shown in FIG. 1, a gigahertz test jig 10 is girded by a ground wall 12 having insulated openings 14 therein. Coaxial RF connectors 16 having inner conductors or probes 18 and coaxially surrounding shells 20 are secured to ground wall 12, with the shells being in electrical contact therewith. Insulation 22 in openings 14 electrically isolates inner conductors 18 from wall 12. Dielectric layers 24, having signal carrying traces or microstrips 26a and 26b and a ground plane layer 28 thereon, are secured to ground wall 12 by ground rings 30. A conductor 32 extends through dielectric layers 24 and electrically couples ground plane 28 to ground rings 30 and, therefore, to ground wall 12. Another conductor 34, which passes through an insulative sleeve 36 in ground plane layer 28, electrically couples microstrips 26a and 26b together. Both microstrips 26a and 26b are mechanically and electrically coupled to inner conductors 18 of external jig connectors 16 by solder joints 38.
In part, the placement of inner conductor 18 and microstrips 26a and 26b was enforced by a requirement accepted in the industry that microstrip bends be kept to less than a 60.degree. bend to avoid excessive insertion losses and reactive impedances from appearing in the transmission lines, which would otherwise cause large changes in the characteristic impedance. Such changes in the characteristic impedance caused large signal reflections and a resultant excessive signal degradation. While 90.degree. bends are known, they were obtainable only for use up to only UHF frequencies where degradation in signal transmission is not a problem.
The connections between microstrips 26a and 26b and conductors 18 and between ground plane 28 and ground wall 12 produced insertion losses, where the electric signals did not follow the desired paths to as great an extent as would be desired. Solder joints 38 did not provide the best mechanical interface connections respectively between microstrips 26a and 26b. Therefore, inner conductors 18 of connectors 16 not only are indirectly coupled to microstrips 26a and 26b but also are exposed to potentially disturbing external signals. Further, because connector 16 is coupled through ground wall 12 to ground plane 28 by an intermediate conductor 32 and the connection between ground plane 28 and wall 12 is a clamp, rather than by a bond, such as by solder, there is a concern that, under some conditions, the electrical connection might become subject to uncoupling or loosening through variations in temperature, vibrations, corrosion, etc.
However well the assembly functions, future market needs make it desirable that its cost of fabrication be reduced to make it more competitive, that its size be reduced, and that its bandwidth be augmented to enable it to operate in excess of 14 GHz and in the x-band frequencies.